1. Field of the Invention
The present invention relates to the field of semiconductor masks and, more particularly to the fabrication of phase shift masks.
2. Background of the Related Art
The use of photomasks to pattern features on semiconductor wafers by employing optical lithography techniques is known in the art. Integrated circuit (IC) devices,; are fabricated from these patterned features. A more recent application is the use of phase shifting techniques to pattern the features. Phase shifting techniques rely on the interference pattern of the projected light to provide or assist in the image formation at the target. See for example, "Improving Resolution in Photolithography with a Phase-Shifting Mask" by Levenson et al.; IEEE Transactions on Electron Devices, Vol. ED-29, No. 12, December 1992, pp. 1828-1836.
In the standard methods of fabricating phase shift masks, 180.degree. phase shifting regions are formed on or in a transparent substrate, such as quartz or glass. In one technique, the light travelling through the deposited material is phase shifted, due to the refractive index of the deposited material being different than that of the substrate. In the second instance, the open area provides a different medium (typically air) than the substrate to shift the phase of the light travelling through it. It is well known that phase shift masks are capable of providing much improved resolution, so that a smaller critical dimension (CD) can be obtained from imaging a pattern as compared to using binary masks.
An improved type of phase shift mask known in the art is shown in FIG. 1. Phase shift mask (PSM) 10 is a three-phase PSM having three separate phase shifting regions. PSM 10 is formed from a transparent mask substrate 11. A light absorbing (or attenuating) patterning layer or element 12 is formed on the surface of the substrate 11. Chrome is a common material used for the element 12 to form an opaque image region. The phase shifting regions are shown within an etched opening 13 formed in the substrate 11.
In the simplest of PSMs, the etched opening 13 has one depth to provide an 180.degree. shift. That is, light travelling through the opening is shifted 180.degree. as compared to light travelling through the full thickness of the substrate 11. This is exemplified by the 180.degree. notation between the surface of the substrate 11 and the depth of the opening 13. However, in the three-phase PSM 10 of FIG. 1, three different phase regions are within opening 13. The deepest portion of the opening 13 still provides the 180.degree. shift. The two shallower regions within the opening provide the other two corresponding phase shifts. In a typical three-phase structure, the phase shifts are 60.degree., 120.degree. and 180.degree.. The surface is referenced as 0.degree., and the phase shifts are referenced to light travelling this path.
As shown in FIG. 2, an advantage of using the three-phase shifter of FIG. 1 is the improved transition from 180.degree. to 0.degree.. FIG. 2 shows the transmitted light intensity through the mask 10 of FIG. 1. The very low or no intensity region 16 corresponds to the light blockage by the light absorbing element 12 and the phase shifting area (shown by intensity profile 17) corresponds to the phase shifting region of opening 13. The light region 18 corresponds to the exposed non-shifted (0.degree.) area of the mask. The three-phase shifts provide for a more gradual transition from low to high intensity (dark to light). The three phase steps within opening 13 prevent formation of a sharp low-to-high light intensity region that would normally occur in a transition from a 180.degree. phase region and a 0.degree. phase region without any graduation. The three-phase mask of FIG. 1 inhibits unwanted printing of lines in photoresist, when the mask is used in lithographic imaging. One example of a three phase PSM is disclosed in U.S. Pat. No. 5,308,722 issued to Nistler.
It is appreciated that the PSM 10 has a structure which requires a graduated opening to be formed in the substrate. Generally, utilizing conventional techniques, three separate etching steps are needed to form the three levels of opening 13. For the etching process, three iterations of: depositing photoresist, patterning the photoresist using photolithography, developing the patterned photoresist to expose the underlying substrate and etching the exposed substrate are required. That is, each depth of opening 13 requires a complete lithographic patterning sequence. Thus, for a three-phase PSM shown in FIG. 1, three separate lithographic steps are required. If additional graduations are desired, additional patterning and etching sequences are needed. The present invention provides for a multiple-phase PSM in which separate lithographic patterning steps are not required for forming everyone of the phase shifting graduations.